Group teleconferencing with loudspeakers and microphones allows a freedom of movement and a degree of interaction among participants that is not possible when conventional telephone handsets are used. Typically, a group teleconference system will have a microphone to receive the speech of various participants in the room and a loudspeaker from which the participants may hear the voices of persons on the other end of the line. Unfortunately, problems are introduced when talkers are several feet away from the microphone and by the acoustical coupling (feedback) from the loudspeaker to the microphone.
Moving a microphone away from a talker's mouth will reduce the level of direct speech pickup relative to room noise and reverberation. This reduces the clarity and intelligibility of the speech and can result in the characteristic "hollow" or "bottom of the barrel" sound often associated with conference sets.
The acoustical coupling from the loudspeaker to the microphone is a problem because it forms a feedback path which, if not controlled, will cause howlback and echo. This is generally not a problem in telephone handsets because the coupling from earpiece to mouthpiece is very small.
The system may be a "four-wire" system, i.e., having separate receive and transmission paths. In such a case, where a conference system exists at both ends, the acoustical coupling of both systems creates a feedback path which can be unstable and causing howling. An echo problem also may occur. If the transmission link has significant time delay associated with it (i.e., about 1/4 second each way for a "single hop" satellite connection), a microphone signal at one end which is acoustically coupled and returned by the far end will be heard by the talker as a delayed echo of his own voice. This can be a significant distraction and can even make conversation impossible.
Also, the system may be a "two-wire" system, i.e., standard telephone lines where the send and receive paths are the same. In such a case, local coupling of the send signal to the receive input completes the acoustic feedback path, again with potential instability an howling.
Because of the above requirements, attempting to maintain "full-duplex" operation) both loudspeakers and microphones fully active at all times) generally requires extensive acoustical treatment of the room, careful loudspeaker and microphone placement, and accepting low, fixed speaker volume settings. Even then, weak received telephone signals could not be brought up in level because of the increased gain inside the feedback loop this would require. Satellite delayed echoes would still make the system unusable.
Heretofore, the need for careful acoustic design, has been reduced by utilizing "half-duplex" operation. Either the microphone or the loudspeaker is always suppressed (attenuated) depending on the assumed direction of conversation. This suppression is typically 40 dB or more.
While this solves the acoustic feedback related problems, it has often done so at the expense of interactive, natural conversation. Beginning syllables and words may be lost; neither end of a conference may be able to interrupt the other; and both ends may be talking without hearing each other.
It is therefore an object of the present invention to provide an improved teleconferencing system which automatically controls feedback loop gains for feedback stability in both four-wire and two-wire applications under changing operating conditions.
It is another object of the present invention to automatically control feedback loop gains for echo control under changing conditions in four-wire applications with transmission links having significant time delay.
It is another object of the present invention to provide an improved teleconferencing system which automatically controls feedback loop gains while still maintaining the capability for fully interactive conversation using conversationally oriented send-receive direction switching with full interrupt capability.
It is another object of the present invention to provide a teleconferencing system which improves sound quality by reducing microphone pickup of room noise and reverberation, optimizing electrical and acoustical frequency responses, and providing manual and automatic loudspeaker level adjustments.
It is another object of the present invention to provide an improved teleconferencing system which facilitates interface with tape players, tape recorders, auditorium sound reinforcement systems, etc.
It is another object of the present invention to provide an improved teleconferencing system having the capability to work in bridged modes, i.e., modes in which more than two locations are interconnected, as for example, conference calls.